To simply answer your title: Yes. You're allowed to do this and plenty of people have done it.
I think he want try to abuse the trust system in this forum.
Trust ratings are not moderated, and if the user is not part of DT it doesn't really matter.
I hope someone will tag this liar.
I do doubt that a DT member will tag him though.
Now I see the clear picture he gave you a red tag because you are a competition and you accuse him of copying your campaign,it i s still not right to give you a fake reviews although you gave him a correct one,it's better deal this issue between yourselves it has nothing to do with scamming ..
Some people are just impossible to work with. OP may as well invest his or her time into something else.